The present invention is directed to a process for preparing a metal/oxygen composition which composition is capable of dehydrocoupling toluene to stilbene.
Styrene is currently commercially produced from benzene in a two-step process. In the first step benzene is alkylated with ethylene to form ethylbenzene, and in the second step, the ethylbenzene is dehydrogenated to form styrene.
One of the known alternative routes for forming styrene involves the oxidative coupling of toluene to form 1, 2-diphenyl ethylene (stilbene) followed by the disproportionation of the stilbene with ethylene in the presence of a catalyst to form styrene. The economic significance of the overall process scheme of the toluene-stilbene route is that styrene can be produced from 0.5 mole of ethylene and one mole of toluene. This compared with the conventional ethylbenzene route wherein styrene is produced from one mole of ethylene and one mole of benzene. In light of the rising costs of benzene and ethylene and the environmental problems of benzene, the toluene-based process will become a more attractive route than the existing benzene-based process for styrene manufacture.
In addition to its utility as an intermediate in production of styrene, stilbene, because of its unsaturated character, is very reactive and may be employed in various organic syntheses. Derivatives of stilbene are useful in the production of chemicals which may be used in the manufacture of dyes, paints, and resins. It is also useful in optical brighteners, in pharmaceuticals and as an organic intermediate.
Thus, there is substantial economic incentive to develop an economical process for producing stilbene.
The oxidative coupling of toluene to 1,2-diphenyl ethane (i.e., bibenzyl) and stilbene has been known for many years.
The ideal reaction to stilbene from toluene is the direct dehydrocoupling reaction summarized as follows: ##STR1## Such a selective reaction in practice is difficult to achieve. More often, the overall reaction involves the dehydrocoupling of toluene to stilbene as well as bibenzyl. Bibenzyl however can be dehydrogenated to stilbene. Thus, a commercial process for producing stilbene could include an overall reaction scheme summarized as follows: ##STR2## although the greater selectivity of the reaction to stilbene, the more efficient the process.
The reaction of Equation 1, employing oxygen as the oxidant in the absence of a catalyst, is extremely inefficient because of the preponderance of non-selective free-radical reactions leading to complete combustion of the hydrocarbons and the formation of oxygenated by-products. Consequently, attempts have been made to improve the selectivity of the reaction using oxidants, such as metal or non-metal oxides as stoichiometric reactants providing lattice oxygen which is depleted during the reaction. Such metal oxides can also function as catalysts for a primary oxidant such as oxygen. Because of the oxygen depletion of metal oxide stoichiometric oxidants, their use requires that they be either very inexpensive and therefore disposable, or they must be capable of being regenerated by replacing the lattice oxygen lost during the reaction. Since many of the conventional stoichiometric metal oxide oxidants are expensive, their use requires extensive plant equipment and engineering design to provide proper regeneration. This has led to two alternative approaches; namely, fixed bed and fluidized bed systems. In the fixed bed system, two or three reactors with staggered cycles typically are employed to achieve continuous operation. This system is very costly in terms of plant equipment. In the fluidized system a single reactor can be employed and a portion of the metal-oxide can be constantly removed, regenerated, and returned to the reactor. Fluidized systems, however, lead to attrition of the metal oxide and in many instances the metal of the metal oxide can be lost as fines which coat the walls of the reactors.
Another difficulty with stoichiometric oxidants and/or catalysts is that those producing relatively good selectivity usually result in a slow reaction rate. In addition the oxygen-carrying capacity is usually very low leading to a short active life.
Representative examples of conventional metal oxide oxidants and/or catalysts are disclosed in U.S. Pat. Nos. 3,694,518; 3,739,038; 3,868,427; 3,965,206; 3,980,580; 4,091,044; 4,183,828; 4,243,825; 4,247,727; 4,254,293; 4,255,602; 4,255,603; 4,255,604; 4,268,703; 4,268,704; 4,278,824; 4,278,825; and 4,278,826. These patents disclose various metal/oxide compositions which can be prepared by a variety of methods. For example, the simplest method involves intimately mixing the powdered metal oxides in the dry state and calcining. Another method involves adding the metal oxides to water with stirring, filtering to remove excess water or, alternatively, heating to evaporate the water, drying, and calcining. In another method of preparation, the powdered metal oxides can be intimately mixed before forming a paste with water and further mixing the paste. The paste can be dried in air, after which it can be calcined in air. The calcined product can then be crushed and sieved to the desired mesh size. In still another method of preparation, the powdered metal oxides can be mixed in the dry state together. A further method of preparation involves intimately mixing the powdered metal oxides in water and spray drying the resulting slurry or solution to produce relatively dust-free and free-flowing spherical particles which are also calcined prior to use. In an alternative method of preparation, suitable inorganic metal/oxygen composition precursor salts such as nitrates, carbonates, and acetates are intimately mixed or dissolved in water or nitric acid and heated to thermally decompose the precursor salts to form the corresponding oxides and/or oxygen complexes. The oxides and/or oxygen complexes can then be treated as described hereinabove prior to use.
Thus, a majority of these preparative methods employ water and are referred to herein as aqueous preparations. None of these patents disclose the use of organic liquids to prepare the metal oxide compositions.
The metal oxide compositions disclosed in these patents prepared by an aqueous method exhibit an extremely short active life. For example, Example 6 in U.S. Pat. No. 4,091,044 illustrates the use of a Sb/Pb/Bi oxide oxidant prepared by the aqueous method. When run for 1 minute at 580.degree. C. (run 3, Table 6) the conversion is 47.3% and a selectivity for cis and trans stilbene plus bibenzyl is 81.2%. However, after 7 minutes reaction time (run 6, Table 6) the conversion drops to 9.7% at a corresponding selectivity of 87.5%. The substantial drop in conversion over a period of 5 minutes indicates that the oxidant is quickly deactivated and implies that it is not an efficient oxygen carrier. It is for this reason that the oxidant is typically regenerated for 30 to 60 minutes after each one-minute run (see Example 1, lines 34 et. seq.). Thus, not only is the oxidant quickly deactivated but its regeneration time is also quite long.
It is known that vanadium phosphorus oxygen catalysts for the oxidation of hydrocarbons, e.g. butane, to form, for example, maleic anhydride, can be prepared using an organic medium, such as isobutanol, as illustrated by U.S. Pat. Nos. 3,864,280; 4,132,670 and commonly assigned U.S. patent application Ser. No. 326,543 filed Dec. 2, 1981. However these catalysts are not employed for the conversion of toluene to stilbene.
The search has therefore continued for metal oxide compositions for use in conjunction with the conversion of toluene by oxidative dehydrocoupling to stilbene which possess the characteristics of (1) high activity and selectivity to minimize toluene recycle and loss to undesired by-products, (2) high oxygen-carrying capacity, (3) high reoxidation or regeneration rate to minimize the amount of composition employed, (4) high attrition resistance under conditions of repeated oxidation and reduction, and (5) high reaction rate. The present invention was developed in response to this search.